How to make a square grid for UV maps
Follow Active Quads will work in this case, but you need to pay attention to a few things to get a clean 90-degree unwrap.
In this method Blender takes the active face and lays out other selected faces around it, following its shape in UV space (not in 3D space). In other words, it is important how your active quad looks in the UV space.
If your mesh doesn't have a UV map (you didn't unwrap it), the active face is mapped to the size of UV space (a perfect square/rectangle) and other faces are placed around it accordingly.
But if your mesh already has a UV map, the active face is not mapped to the size of UV space, but rather its existing shape in UV space is used.
For example, if your UV map looks like this (notice the shape of the active quad):

the result of unwrapping with Follow Active Quads will look like this (notice how selected faces are following the shape of the active face):

To get a nice grid, you have to turn your active quad into a perfect rectangle in UV space. You can do that by editing the vertices manually, by resetting the UV map (U &gt; Reset), or by using Lightmap Pack unwrap (U &gt; Lightmap Pack &gt; OK). I usually use Lightmap Pack unwrap in cases like this. It's fast and it fits all the faces into UV space (they are smaller, so after using Follow Active Quads the unwrapped grid isn't huge).
The result of using Lightmap Pack will look like this (notice the active face turned into a perfect rectangle).

Now it's OK to use Follow Active Quads - you should have a nice 90-degree grid:



To sum it up, here's your workflow:

Select all faces you want to unwrap.
Use Lightmap Pack unwrap (U &gt; Lightmap Pack &gt; OK).
With active quad selected use Follow Active Quads (U &gt; Follow Active Quads).